1. Field of the Invention
The present invention relates to a film stretching apparatus and a film stretching method for producing a high-functional film, and especially to a film stretching apparatus and a film stretching method for producing a wide variety of an optical films in small quantities.
2. Description Related to the Prior Art
A polymer film used in an optical field is produced by a melt-extrusion method or a solution casting method. In the melt-extrusion method, powder or pellets of polymers are heated and melt, and the melt polymer is extruded from a die to form a thin film. Then the thin film is cooled so as to be the film. In the solution casting method, the polymer and the solvent are mixed so as to be a dope, which is cast onto a support from a die to form a casting film. Having the self-supporting properties, the casting film is peeled, and dried so as to be a film. The support is a drum or belt that continuously runs.
There are several sorts of the polymer for the film. Especially, cellulose acylate is often used as the polymer in an optical field, since cellulose acylate has transparency, adequate moisture permeability, large mechanical strength, and low dependence of dimensional resistances on moisture and temperature. When the cellulose acylate is used as the raw material of the film, the solution casting method is often applied to the film production.
In the solution casting method, a continuous film is which the solution casting method on the prior art is applied will be explained in reference with FIG. 17. A film production equipment 200 is a continuous type and includes a casting apparatus 201, a dry-stretching device 202, a roller-drying apparatus 203 and a winding apparatus 204.
The casting apparatus 201 includes a support 206 and a casting die 207. The dry-stretching device 202 is called a tenter device having an air-conditioner (not shown) for air ventilation in the dry-stretching device 202. Further, a necessary number of rollers 209 for supporting the film is provided between or in the devices or apparatuses.
In the casting apparatus 201, a dope 212 is cast from the casting die 207 onto a support 206 such as the drum and the like, so as to form a casting film. Then, from the support 206, the casting film is peeled as a film 213 with content of the solvent. The film 213 is transported to the dry-stretching apparatus 202 which includes drying sections A1-An. In each of the drying sections A1-An, a drying air is fed toward the film 213. The temperature and the wind speed are controlled in each drying section A1-An. Further, in the dry-stretching apparatus 202, the film 213 is transported with holding both side edge portions of the film 213. Note that a holding device (not shown) is a tenter clip or a holding pin. The dry-stretching apparatus 202 stretches the transported film in a predetermined direction while drying. The holding apparatus releases the film 213 at a predetermined position near an exit of the dry-stretching apparatus 202.
After the dry-stretching apparatus 202, the drying apparatus 203 dries the film 213 with transportation with use of the roller 203a and the like, and thus the solvent in the film 213 evaporates. A surface treatment device may be provided in the drying apparatus 203 for performing a predetermined surface treatment. The air conditioner includes a gas concentration sensor and a temperature sensor for respectively detecting a gas concentration in the dry-stretching apparatus 202 and the drying apparatus. The air conditioner further includes a controller (not shown) for controlling the drive of the air conditioner on the basis of the results of the gas concentration sensor and the temperature sensor. Thus the concentration of the solvent gas and the temperature in the dry-stretching apparatus 202 and the drying apparatus 203 are controlled. In the winding apparatus 217, the both side edge portions are slit off such that the dried film 213 may have a predetermined width, and thereafter the film 213 is wound. The continuous solution casting equipment is adequate for mass production in which the conditions for film production is uniform, as described in Japanese Patent Laid-Open Publication No. 2002-265636.
Recently, however, it is necessary to produce many sorts of the films in accordance with use or request of the users, and further, the quantities of the produced film is sometimes small. In the case of the production of the small quantities of the films, the continuous solution casting as described in the publication No. 2002-265636 has following problems:                (1) Since the quantity of the materials to be prepared is much larger than the corresponding quantity to each sort of the film to be produced:        (2) It is hard to change the producing condition, especially the drying condition corresponding to the sort of the film to be produced, and a long time is lost for changing the conditions. Further it is hard to change the conditions substantially:        (3) Even if the production number is small, the film production is stably made under basic conditions, and thereafter the conditions are changed to those for producing the objected film. In this case, it takes long time until the film production is stably made after the change of the condition for the objected film production. Further, before the production after the change becomes stable, the film splits. In this case, many people and time are necessary for recovery of the running:        (4) The stretching direction in the dry-stretching apparatus is restricted in the widthwise direction or the lengthwise direction:        (5) Even if it is designated to change the drying condition depending the sorts of the film, the drying conditions are restricted because of the structure of the equipment:        (6) The equipment is large.        